Steam humidification systems are commonly used to raise the humidity level in air flow ducts. Typical untreated air in the winter months has very low relative humidity, and it is desirable to increase the level of humidity in commercial and industrial facilities. This is particularly true for health care facilities such as hospitals and nursing homes. High relative humidity is also needed in industrial locations where static electricity is especially undesirable, such as in facilities housing electronic equipment, and in other industrial locations, such as fabric or paper handling, where a material must be prevented from drying out.
Steam humidification systems typically use dispersion tubes that are supplied with steam and have numerous orifices to discharge steam. Usually the dispersion tubes are positioned within air handling systems such as heating, ventilating and air conditioning ("HVAC") ducts to discharge steam into the air flowing through the ducts. Since the steam is warmer than the air flowing through the HVAC ducts, the air flow in the ducts has a cooling effect on the dispersion tubes, and as the steam enters the dispersion tubes, some of the steam is cooled to the extent that it condenses into water. This is to be avoided because the water can be discharged through the discharge orifices in liquid form along with the steam in vaporous form. The result is excessive dampness in the HVAC duct and other equipment, thereby providing an environment ripe for the growth of undesirable microorganisms.
Designers of steam humidification systems know that the tendency of steam to condense in the dispersion tube can be counteracted by providing a heated jacket around the dispersion tube to help maintain the dispersion tube warm enough so that condensation does not occur. A flow of steam through the jacket passageway keeps the dispersion tube from cooling off, thereby minimizing condensation in the dispersion tube. This is illustrated in U.S. Pat. No. 3,857,514 to Clifton, which shows a steam supply directed to the jacket around a dispersion tube. The steam exiting the flow-through jacket is directed to a steam control unit, where the steam is conditioned to remove condensed water and mist. Then the conditioned steam is delivered to the dispersion tube for discharge into the HVAC duct. Steam humidification systems are configured to supply steam to the jacket at a pressure within the range of from about 2 to about 60 pounds per square inch ("psi"), in order to ensure a good flow through of steam through the jacket and through the steam control unit, although a pressure within the range of from about 10 to about 15 psi is typical.
Another known system for the humidification of a flow of air is the use of an array or bank of unjacketed dispersion tubes that are enhanced with various features to prevent condensed water from being discharged from the discharge orifices along with the steam. U.S. Pat. No. 5,516,466 to Schlesch et al. shows such a system, where the discharge orifices have tiny tubes that extend into the dispersion tube to prevent condensed water from easily exiting the dispersion tube along with the flow of humidification steam. This patent also discloses baffle tubes that help increase the velocity of the air at the point of steam mixing to improve vaporization of any visible vapor exiting the dispersion tubes.
It would be desirable to be able to provide a jacketed dispersion tube humidification system where there is no requirement for a return line for steam passed through the jacket. This would substantially reduce the steam piping requirements. However, it would be expected that a jacketed dispersion tube, where there is no return for a flow-through of steam, would be unworkable. The steam would flow at a slow rate because the primary mechanism for inducing flow into the jacket is the condensation of the steam within the jacket rather than a flow through of the steam. This condensation causes a reduction in volume and resulting influx of a small amount of additional steam. A condensate return line could be provided to drain any condensed water from the bottom of the dispersion tube. Because of the low steam throughput in the jacket, such a jacketed dispersion tube would be expected to be only partially successful in maintaining the dispersion tube warm enough to provide dry steam, i.e., steam having little condensed water or mist.
Another problem to be expected with jacketed dispersion tubes designed with no flow-through is that non-condensable gases, such as air and carbon dioxide, would accumulate in the jacket. This accumulation of non-condensable gases is referred to as "air binding". Since, unlike steam, these gases cannot condense to make room for additional steam, the flow of additional hot steam into the jacket would be greatly diminished. The resulting extremely low steam flow in the jacket would be expected to prevent effective heating of the dispersion tube, and excessive unwanted condensation would be expected to occur in the dispersion tube. It would be beneficial if a jacketed dispersion tube, having no return for a flow-through of steam, could be made to successfully distribute humidification steam.
Standard humidification systems using jacketed dispersion tubes have in the past been configured to be connected to a source of high pressure steam, such as an industrial steam boiler. This requires running a steam line from the boiler to the location where the steam is to be introduced into the HVAC duct. Improvements in steam generating equipment have resulted in the availability of independent steam generators capable of being positioned close to the location where the steam is to be introduced into the HVAC duct. These independent steam generators can be, for example, electronic steam humidifiers, gas-fired steam humidifiers, or steam to steam humidifiers.
One of the characteristics of independent steam humidifiers is that they generate steam only at low pressures, usually no greater than one psi, and typically significantly less than 27 inches of water column. With such low steam pressures, it is not feasible to operate a typical jacketed steam dispersion tube of the type disclosed in U.S. Pat. No. 3,857,514 to Clifton, as described above. At such low initial pressures, the steam would not be expected to be capable of flowing through a stream control unit for conditioning. Therefore, supplying traditional jacketed dispersion tubes with low pressure steam from a low pressure steam humidifier has not been very successful. It would be advantageous if jacketed steam dispersion tubes could be operated using the steam from a low pressure steam source such as an electronic steam humidifier.